The present invention relates to subterranean treatment operations, and more particularly, to improved bridging agents comprising a degradable material, to improved subterranean treatment fluids comprising such improved bridging agents, and to methods of using such improved subterranean treatment fluids in subterranean formations.
A subterranean treatment fluid used in connection with a subterranean formation may be any number of fluids (gaseous or liquid) or mixtures of fluids and solids (e.g., solid suspensions, mixtures and emulsions of liquids, gases and solids) used in subterranean operations. An example of a subterranean treatment fluid is a drilling fluid. Drilling fluids are used, inter alia, during subterranean well-drilling operations to, e.g., cool the drill bit, lubricate the rotating drill pipe to prevent it from sticking to the walls of the well bore, prevent blowouts by serving as a hydrostatic head to counteract the sudden entrance into the well bore of high pressure formation fluids, and also remove drill cuttings from the well bore. Another example of a subterranean treatment fluid is a “drill-in and servicing fluid.” “Drill-in and servicing fluids,” as referred to herein, will be understood to include fluids placed in a subterranean formation from which production has been, is being, or may be cultivated. For example, an operator may begin drilling a subterranean borehole using a drilling fluid, cease drilling at a depth just above that of a potentially productive formation, circulate a sufficient quantity of a drill-in and servicing fluid through the bore hole to completely flush out the drilling fluid, then proceed to drill into the desired formation using the well drill-in and servicing fluid. Drill-in and servicing fluids often are utilized, inter alia, to minimize damage to the permeability of such formations.
Subterranean treatment fluids generally are aqueous-based or oil-based, and may comprise additives such as viscosifiers (e.g., xanthan) and fluid loss control additives (e.g., starches). Subterranean treatment fluids further may comprise bridging agents, which may aid in preventing or reducing loss of the treatment fluid to, inter alia, natural fractures within the subterranean formation. Calcium carbonate is an example of a conventional bridging agent. In certain circumstances, a bridging agent may be designed to form a filter cake so as to plug off a “thief zone” (a portion of a subterranean formation, most commonly encountered during drilling operations, into which a drilling fluid may be lost). Generally, bridging agents are designed to form fast and efficient filter cakes on the walls of the well bores within the producing formations to minimize potential leak-off and damage. Generally, the filter cakes are removed before hydrocarbons are produced from the formation.
Conventionally, the filter cakes are removed from well bore walls by contacting the filter cake with one or more subsequent fluids. For example, where an aqueous-based treatment fluid comprising bridging agents is used to establish a filter cake, operators conventionally have employed enzymes and oxidizers to remove the viscosifier and fluid loss control additive, and then used an acid, or a delayed-generation acid, to clean up the calcium carbonate bridging agent. The removal of filter cakes established by oil-based treatment fluids, however, is often much more difficult.
When an oil-based treatment fluid comprising bridging agents is placed in a subterranean formation, a filter cake often results that covers the walls of the well bore. Because the fluids that subsequently will be placed in the well bore often will be aqueous-based, an operator ordinarily might prefer to remove this filter cake with an aqueous-based cleanup fluid that may be compatible with the subsequent fluids. However, attempts to remove the filter cake with an aqueous-based cleanup fluid generally have been unsuccessful, due at least in part to the fact that oil and water are immiscible, which may impair the aqueous-based cleanup fluid's ability to clean the filter cake off the well bore walls. Accordingly, operators have attempted to introduce acid into the well bore, to try to dissolve the calcium carbonate bridging agents which are acid-soluble. This method has been problematic, however, because such calcium carbonate bridging agents are generally well-mixed within the filter cake. Multi-stage cleanup operations usually ensue, and may include, in a first stage, the introduction of water-wetting and oil-penetrating surfactants, followed by multiple stages that involve the introduction of an acid solution into the well bore. Additionally, some operators have attempted to use an oil-based treatment fluid having a particular pH to establish a filter cake (which, as noted above, is essentially a water-in-oil emulsion when formed by an oil-based treatment fluid), and followed the oil-based treatment fluid with a cleanup fluid having a pH that is sufficiently different to invert the emulsion (e.g., the filter cake) to become water-external, thereby water-wetting the bridging particles within the filter cake.
These conventional methods have been costly, laborious to perform, and generally have not produced the desired results, largely because the filter cake is not cleaned evenly-rather, the cleanup methods described above generally only achieve “pinpricks” in the filter cake itself. These pinpricks may be problematic because the well bore is typically under hydrostatic pressure from the column of treatment fluid, which may be lost through these pinpricks where the filter cake has been penetrated. Thus, any fluid that subsequently is placed within the well bore may be lost into the formation, as such fluid may follow the path of least resistance, possibly through the pinpricks.